This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the presently disclosed inventions. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the presently disclosed inventions. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Some approaches to lower carbon dioxide (CO2) emissions include fuel de-carbonization or post-combustion capture. These solutions, however, are expensive and reduce power generation efficiency, resulting in reduced power production, increased fuel demand, and increased cost of electricity to meet domestic power demand. Another approach is an oxy-fuel gas turbine in a combined cycle. However, there are no commercially available gas turbines that can operate in such a cycle.
The oxy-fuel concept is based on the combustion of hydrocarbons with pure oxygen (O2) to produce carbon dioxide and water (H2O). Such a combustion process, however, produces extremely high temperatures that reduce combustor life and generate soot and other unwanted combustion products. Hence, a cooling gas of some kind is desirable.
Various cycles have been proposed and studied that use carbon dioxide or steam as a mass flow gas through the turbine instead of air. Some basic laboratory experiments have been undertaken to better understand the physics of the combustion process in these oxy-fuel arrangements. While some experimental progress has been made in steam-based oxy-fuel arrangements, the design and implementation of an oxy-fuel gas turbine with a carbon dioxide working fluid for commercial applications has not been achieved. The design of a combustor for a carbon dioxide type gas turbine has never progressed beyond lab-scale experiments.
Challenges related to the design and implementation of carbon dioxide and oxygen mixing and combustion in a practical gas turbine combustor have not previously been addressed. Unlike steam, carbon dioxide has an inhibiting effect on the combustion process, which requires a unique design to handle the lower flame speeds resulting from the inhibiting effect. Carbon dioxide also radiates more energy than nitrogen or steam, which leads to the potential for preheating the reactants via radiative heat transfer. There is also an additional degree of freedom in an oxy-fuel combustor since the oxygen-to-fuel ratio can be controlled independently from the flame temperature, which is primarily dependent on the oxygen-to-carbon dioxide ratio.
Because of the additional degree of freedom in oxy-fuel combustion systems, the flow rate of oxygen can be controlled independently from the inert diluent (steam or carbon dioxide). This is not the case in a typical air gas turbine where there is a fixed ratio of approximately 3.76 inert nitrogen molecules for each oxygen molecule in the oxidizer stream. Another challenge of the oxy-fuel combustor is that oxygen is a precious commodity and must be obtained from any number of expensive, energy intensive processes, such as an air separation process, a special membrane separator, or some other process such as electrolysis of water. Typical air gas turbines have an air flow path that is designed to split the air stream such that a portion is used for the combustion reaction and a second portion is used for cooling of the combustion products and the combustion liner. This results in an exhaust stream that contains more than 10% oxygen.
Commonly assigned PCT Patent Publication No. WO2010/044958, which is incorporated herein by reference in its entirety for all purposes, discloses methods and systems for controlling the products of combustion using a system of flow controllers and sensors to maintain stoichiometric combustion. However, that disclosure does not provide details of the configurations in the combustor.
There is a need, therefore, for improved systems and methods for obtaining substantially stoichiometric combustion in an oxy-fuel type combustion reaction.